Update plugin format + tests

This fixes #4 by using the newer plugin format for python-novaclient. This also enables it to get auto-loaded.

Before:

```python
In [1]: from novaclient import auth_plugin

In [2]: auth_plugin.discover_auth_systems()

In [3]: auth_plugin.load_plugin('rackspace')
Out[3]: <novaclient.auth_plugin.DeprecatedAuthPlugin at 0x10acc15d0>
```

After:

```
In [1]: from novaclient import auth_plugin

In [2]: auth_plugin.discover_auth_systems()

In [3]: auth_plugin._discovered_plugins
Out[3]: {'rackspace': rackspace_auth_openstack.plugin.RackspaceAuthPlugin}

In [4]: auth_plugin.load_plugin('rackspace')
Out[4]: <rackspace_auth_openstack.plugin.RackspaceAuthPlugin at 0x103942590>
```

Additionally, this PR adds tests, travis support, and an update to the changelog.

This also adds `python-novaclient` as a dependency so that we can use the plugin base class.